1. Field of the Invention
The invention relates generally to an apparatus for retaining a workpiece within a semiconductor wafer processing system and, more specifically, to an improved composition of a polyimide based electrostatic chuck that maximizes electrostatic clamping ability without loss of material strength or modulus of elasticity.
2. Description of the Background Art
Electrostatic chucks are used for retaining a workpiece in various applications including retaining a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. The pedestal may form an electrode and a heat sink or heater as used in etching, physical vapor deposition (PVD) or chemical vapor deposition (CVD) applications. For example, FIG. 1 depicts a cross-sectional view of a reaction chamber used in semiconductor wafer processing. For a detailed understanding of the reaction chamber and its operation in processing the wafer, the reader should refer to the drawings and the detailed description contained in U.S. Pat. No. 5,228,501, issued Jul. 20, 1993, incorporated herein by reference. That patent teaches a PVD wafer processing chamber manufactured by Applied Materials, Inc. of Santa Clara, Calif. Additionally, the operation of a conventional electrostatic chuck is disclosed in U.S. Pat. No. 5,350,479 issued Sep. 27, 1994 to the assignee hereof, and its disclosure is incorporated herein by reference.
The chamber 100 contains a pedestal 106 supporting an electrostatic chuck 104. The electrostatic chuck 104 has at least one electrode 116 which is insulated from a wafer 102 placed upon an upper surface 105 of the electrostatic chuck 104. Specifically, the electrode 116 is either embedded within the body of the electrostatic chuck 104 or encased in layers of dielectric material which comprise the electrostatic chuck. The electrode(s) 116 are coupled to a power supply (not shown) via electrical conductors 118. The voltage from the power supply creates the electrostatic (or clamping) force which draws the wafer 102 to the chuck 104. Additionally, a variety of components may circumscribe the pedestal 106 to protect the wafer 102 and chamber 100 from improper or excessive deposition, etching or the like. Specifically, a deposition ring 108 contacts the edges of the wafer 102 and a deposition shield 124 circumscribes the deposition ring 108 to define a reaction zone 126. Lift pins 110 are mounted on a platform 112. The platform is coupled to an actuator shaft 114 located below the pedestal 106. The lift pins 110 engage the wafer and lift it off the pedestal 106 after processing is completed.
The mechanism of attraction in the electrostatic chuck used in these types of wafer processing systems is generally Coulombic force. That is, the increase of charges in the insulated electrode 116 induce opposite charges to gather on the backside of the wafer. The resultant force is generally weak per unit area i.e., 15 g/cm.sup.2 at 1500V DC because of the composition of the chuck. For example, a commonly used type of dielectric material for fabricating electrostatic chucks is polyimide. Specifically, electrodes are usually sandwiched between two sheets of polyimide to form an electrostatic chuck. Among the beneficial characteristics of polyimide are its high strength and high modulus of elasticity. This material also has high volume resistivity (on the order of 10.sup.14 ohm-cm) and surface resistivity (on the order of 10.sup.14 ohm/cm.sup.2). Since the electrode(s) are insulated and a high resistivity dielectric is used, the charges creating the chucking force are not mobile i.e., the electrode and wafer are separated by the dielectric layer. As such, the wafer must come into contact with a large area of the chuck so that an adequate charge accumulation is established for wafer retention.
Additionally, the backside of the wafer 102 and the top surface 105 of the electrostatic chuck 104 are relatively smooth. However, imperfections in each of these surfaces create interstitial spaces when these surfaces come into contact. As such, not all of the wafer is in direct thermal contact with the chuck. Maintaining a uniform temperature across the entire wafer is essential to proper wafer processing. To maintain proper thermal transfer conditions at the wafer during processing, an inert thermal transfer gas is pumped into the interstitial spaces or specially formed grooves in the chuck surface when the clamping force is applied. More specifically, a feed-through pipe 122 in the pedestal 106 provides thermal transfer gas to an aperture 120 in the top surface 105 of the electrostatic chuck 104. The gas, usually Helium or Argon, acts as a thermal conduction medium between the wafer 102 and the chuck 104 that has better thermal transfer characteristics than the vacuum it replaces. To further enhance thermal transfer conditions (i.e., cooling or heating of the wafer), the pedestal temperature is typically controlled using water-cooled conduits within a cooling plate (not shown) below the chuck 104 and/or with resistive heating elements buried in or clamped to the chuck 104. This cooling technique is known as backside gas cooling.
Since the distribution of thermal transfer gas to the interstitial spaces and chuck groove is osmotic and the interstitial spaces may not all be interconnected, some spaces do not receive any gas. This condition can also lead to a non-uniform temperature profile across the backside of the wafer 108 during processing and result in wafer damage. As such, it is advantageous to have as large a gas aperture and groove width as possible to maximize thermal transfer gas flow and pressure beneath the wafer. However, the limited attractive wafer clamping (Coulombic) force establishes a limit on the size of this aperture and the gas pressure therein. Additionally chuck groove width is limited to approximately 1-2 mm. Specifically, if the thermal transfer gas pressure becomes greater than the Coulombic chucking force, the wafer may shift on the pedestal thereby causing a processing anomaly on the wafer. In an extreme situation, the wafer may even pop off the pedestal onto the chamber floor and likely break, rendering the wafer useless. Since effective and uniform heat conduction away from and/or into the wafer is an important aspect of the manufacturing process, different types of chucks are designed in an attempt to maximize clamping force and thermal transfer.
One example of an improved electrostatic chuck is one that employs the Johnsen-Rahbek (J-R) effect. In such a chuck, the dielectric material has an intermediate resistivity instead of a high resistivity. As such, there are mobile charges present in the dielectric material. These mobile charges create a small but effective current flow between the backside of the wafer and the top surface of the electrostatic chuck. Specifically, at points where these two surfaces come into contact, a zero potential exists. These contact points are extremely small in comparison to the total area of a wafer being retained on the chuck. As such, not all of the mobile charges are able to pass through the contact points. The resultant movement and accumulation of the mobile charges within the top surface of the electrostatic chuck and the backside of the wafer creates a very high electrostatic force across the interstitial spaces between the surfaces. This electrostatic force clamps the wafer to the chuck.
Electrostatic chucks using the J-R effect are usually fabricated from a ceramic having an intermediate or "leaky" dielectric characteristic. Materials such as aluminum and silicon oxides and nitrides are popular and well known for use in electrostatic chucks. However, these types of materials must be carefully machined when creating the gas aperture or similar openings grooves or features; otherwise, they may fracture and become unusable. Additionally, the different coefficients of thermal expansion of the wafer and the ceramic may contribute to the phenomenon of "microgrinding" during processing. Microgrinding causes minute contaminant particles from the surface of the electrostatic chuck to become embedded on the backside of the wafer. Such particles may also be released in the process chamber and contaminate succeeding wafers. Polyimide, exhibits none of the undesirable microgrinding or fracturing characteristics of ceramics. Unfortunately, polyimide exhibits only high resistivity characteristics which is not useful in establishing the J-R effect.
Therefore, there is a need in the art for an improved apparatus for retaining a wafer having a strength and modulus of elasticity comparable to polyimide, but have a reduced resistivity level so as to take advantage of the J-R effect for clamping the wafer. Additionally, such an apparatus must be simple and cost-effective in design and construction to allow for optimal thermal transfer gas aperture and groove size in the apparatus and flow of the thermal transfer gas beneath the wafer.